Organic electroluminescent elements have attracted attention as a next-generation illumination light source because they are thin and enable planar light-emission, and have been vigorously developed with the aim of practical use. In particular, research and development has been actively conducted especially with regard to technologies for improving color rendering properties, which is regarded as one of problems with inorganic LED lighting, and methods for improving color rendering properties by various design and technological developments of devices have been proposed. However, problems remain in order to realize sufficient improvement in color rendering properties in comparison with conventional main lighting, that is, fluorescent lamps.
In WO 2010/134352 A1 (Patent Literature 1), a method is proposed for obtaining a high performance white element by stacking a first light-emitting unit which causes the TTF phenomenon and a second light-emitting unit. The method is proposed as a highly effective method for realizing a white organic electroluminescent element having high efficiency and long life in comparison with a conventional white element in which a first light-emitting unit (causes no TTF phenomenon) and a second-emitting unit are stacked. However, a method for improving color rendering properties which is important for a light source for lighting applications is not referred to.
For lighting purposes, light sources that emit light with various color temperatures are required. Therefore, organic electroluminescent elements having different color temperatures have been developed. However, to produce the organic electroluminescent elements with different color temperatures used for lighting, it is necessary to change light-emitting materials and significantly change the device structure, for example. Particularly in a multiunit structure including the first light-emitting unit and the second light-emitting unit which are stacked, efficiencies of the respective units are likely to be unbalanced. Hence, it is difficult to realize emission of light with various color temperatures while keeping high efficiency and life property, unless the material is changed or the structure is significantly changed.